The present invention relates to circuits for transmitters of radio communications, below denoted as “transmission circuits”.
In general, to process modulated signals with amplitude modulation, especially multiple modulation such as Quadrature Amplitude Modulation (QAM), radio-frequency power amplifiers in transmission circuits for transmitting power to antennas need linear amplification. Therefore, class A or AB had been adopted as operation classes of the RF power amplifiers.
However, with the progress of broadband communication, communication methods using subcarrier modulation such as Orthogonal Frequency Division Multiplex (OFDM) have come to be used and high efficiency is not expected any more with the conventional class-A or AB radio frequency amplifiers. That is, in the OFDM modulation, subcarriers are overlapped so that a large amount of power is instantaneously generated at random time and the ratio between the average power and the peak power, i.e., a Peak to Average Power Ratio (PAPR), is high. Therefore, a large amount of DC power always needs to be held in order to allow linear amplification of peak power which is much greater than the average power. With respect to class-A operation, the efficiency is only 50% at the maximum. In particular, in the case of the OFDM modulation, since the PAPR is high, DC power obtained by multiplying current by the difference between the peak voltage for the peak power and the instantaneous voltage for the instantaneous power is lost in the form of heat in almost all the time except for the period in which the peak power is output. Accordingly, the resultant efficiency decreases largely.
As a result, portable wireless equipments using batteries as their power supplies, for example, have shorter continuous operation time, and thus inconveniences arise in actual application.
To solve this problem, a conventional Envelope Elimination and Restoration (EER) technique known as Kahn Technique was disclosed in FIG. 6 of U.S. Pat. No. 6,256,482B1, for example.
In the configuration disclosed in the above patent (see FIG. 6), an input RF modulated signal is detected and divided into two components. One of the components is an amplitude component corresponding to the envelope of the modulated signal. This amplitude component is subjected to an amplitude modulation by an amplitude modulator constituted by, for example, a switching regulator, and is supplied to a power-supply-voltage terminal of an RF power amplifier. The other component is controlled by an amplitude control amplifier (limiter) to have a constant amplitude, and serves a phase modulated signal (phase component) whose phase only is modulated. This phase component is supplied to an RF input terminal of the RF power amplifier.
According to the EER technique, a switching amplifier with high efficiency can be used as the RF power amplifier so that a minimum power supply voltage necessary for power amplification is supplied to the power-supply-voltage terminal of the RF power amplifier, thereby enhancing the efficiency.
As disclosed in Japanese Laid-Open Publication No. 3-34709 (see FIG. 1), for example, another EER technique is proposed. This EER technique is suitable for digital signal processing and obtains a phase modulated signal by a quadrature modulation of a complex envelope signal. In the configuration disclosed in this publication, a modulated signal with a remaining modulated amplitude is supplied as a phase modulated signal to an RF power amplifier.
FIG. 13 is a block circuit diagram schematically showing a conventional transmission circuit using an EER technique. This transmission circuit includes: a modulated signal generator 101 for outputting a modulated signal to two branched lines; an envelope detector 102 for receiving one of the branched modulated signals, detecting the envelope of the received signal and outputting an amplitude component thereof; a power-supply-voltage generator 103 (a DC-to-DC converter) for receiving the amplitude component from the envelope detector 102 and generating a power supply voltage in accordance with the amplitude value; a phase detector 104 (an amplitude control amplifier) for receiving the other one of the branched modulated signals and outputting a phase modulated signal as a phase component; a quadrature modulator 105 for receiving the phase modulated signal from the phase detector 104 and performing a quadrature modulation thereon; and an RF power amplifier 106 for receiving the output from the power-supply-voltage generator 103 at its power-supply-voltage terminal and receiving the output from the quadrature modulator 105 at its RF input terminal.
The modulated signal generator 101 performs a modulation such as QAM or OFDM modulation based on data generated inside the circuit or supplied from the outside and outputs a modulated signal for transmission represented by a complex envelope. The envelope detector 102 obtains the absolute value of the complex envelope representing the modulated signal and outputs an amplitude component. The power-supply-voltage generator 103 performs DC-to-DC conversion, thereby generating a power supply voltage in accordance with the amplitude component. The phase detector 104 keeps the absolute value of the complex envelope at a constant value while maintaining the phase of the complex envelope representing the modulated signal, and outputs a phase component represented by the complex envelope. The quadrature modulator 105 performs a quadrature modulation on the phase component represented by the complex envelope and converts the component into an RF signal, thereby generating a phase modulated signal. The RF power amplifier 106 amplifies the phase modulated signal (phase component) to obtain an RF signal with an amplitude according to the amplitude modulated voltage (amplitude component), thereby outputting a modulated signal whose amplitude and phase are variable.